Frequency modulation



Dec. 24, 1940. J. L. FlNCH FREQUENCY MODULATION Filed April.2l, 1938 2 Sheets-Sheet 1 T0 AMPLIFIER AND FREQUENCY MULT/PL/ER CIRCUITS 0F TRANSMITTER 1 1 1 1 1 I 1 1 1 1 1 1 I 1 1 1 1 I q 1 1 4 4 1 4 bbbb up I NV EN TOR.

J L. F/NCH ATTORNEY.

J. L. FlNCH FREQUENCY MODULATION Dec. 24, 1940.

' Filed April 21', 1938 2 Sheets-Sheet 2 MODUMT/ON mpur MARK= /80K SPACE 15141 KEYED TONE INPUT TONE SIGNAL CONVERTER I NV EN TOR. K2455 L F/NCH ATTORNEY.

Patented Dec. 24, 1940 FREQUENCY MODULATION James L. Finch, Patchogue, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 21, 1938, Serial No. 203,272

22 Claims.

The present application concerns a new and improved means for frequency modulating wave energy. More in particular this invention concerns means for modulating wave energy in frequency in accordance with signals, comprising a frequency determining circuit in which oscillatory energy flows, a-transmission line connected at one terminal to said frequency determining circuit and at the other terminal to a variable resistance. The character of the transmission line, frequency determining circuit, and resistance is such that when the resistance is varied at one end of the line a variable reactive effect appears at the other end of the line and in the frequency determining circuit. This change in reactance modulates the frequency of the oscillatory energy flowing in the circuit. The terminal variable resistance is modulated in accordance with the signals it is desired to communicate with.

The present invention is simple in nature and structure, economical in operation and effective in accomplishing frequency modulation which accurately characterizes the signals to be transmitted.

In describing my invention reference will be. made to the attached drawings wherein Figures 1 to 4 inclusive each show a modulation system including the features mentioned above and others. Each modification comprises an oscillator and a line or lines coupling resistances variable at signal frequency to reactive elements in the oscillator to control the frequency of oscillation thereof in a new and improved manner without excessively varying the amplitude of the oscillations generated.

Figure 1 shows a specific embodiment of this invention as applied'to a crystal oscillator. In this figure, I is a crystal oscillator, I8 is a line coupled at one end to said oscillator while K is a modulator coupled to the other end of said line. Returning to the oscillator I, 2 is a piezo-electric crystal which is connected between grids 3 and 4 of a multi-grld tube l5. As far as the oscillator is concerned grid 4 acts as an anode and grid 3 acts as the control grid. A screen grid 5 is placed between these two grids for the purpose of avoiding undesired electrostatic capacity between grids 3 and 4. This oscillator circuit is equivalent to the well known Hartley oscillator in which the crystal with its air gap serves as the inductance and the capacity C between grid 3 and ground in series with the capacity 0' between grid 4 and ground serves as the capacity. In this specific application the oscillating portion of the unit is electron coupled to the output portion. The output circuit is comprised of plate II and its tuned circuit l2 and i3 from whichoutput power is obtained. Capacity coupling between the oscillatory and the output circuit is minimized by grids 9 and I0 which are placed between grid 4 and 5 plate il.

In connection with my invention additional circuits are connected at point It to those already described. These circuits consist of blocking condenser ll, 9. section of transmission line l8, a 10 tunable circuit consisting of inductance l9 and capacity 20. Shuntedacross this tunable circuit is the output of vacuum tube 2|, in this case shown as a triode. The grid 22 of this tube is modulated by means of telegraph key 23 asso- 5 ciated with grid resistor 21 and bias voltage battery 25. When this key is open the grid 22 chtains a working bias voltage through resistor 24 from a point 26 on the bias battery of relatively low negative voltage. When the key is closed a 20 blocking potential from point 21 on the bias battery is applied to the grid 22.

The transmission line l8 in this modification consists of a long helix of wire 22 placed near a grounded metal sheet or within a grounded metal 25 tube or other container. 28 has distributed capacity to this grounded sheet or container represented by condensers 29. This line is thus what might be called an artificial line. In this application an artificial type of line is chosen in 30 order to obtain a higher surge reactance than could be obtained with a natural line. The length of this line is adjusted for substantially 5 wave length at the frequency of operation of the osciilator i with which it is used.

In the operation of this invention tuned circuit I9, 20 is adjusted for a slightly higher frequency than that at which the oscillator l is oscillating. Thus, it terminates-line l8 in a reactance which is largely inductive. The line is 40 tapped down on coil I! to a point which will give a suitable value of terminating reactance. When tube 2| is biased to cutoii' by closing switch 23 the impedance in which the line is terminated is practically all inductive reactance. Its value 45 can be adjusted as indicated until the terminating reactance matchesthe surge impedance of the line. When this is done the line will present to the oscillator at point It a very high impedance and its effect on the oscillator will be 5 negligible.

Now assume that the bias voltage on grid 22 is adjusted until a. substantial plate current can flow .inthis tube. This will make this tube conductive of alternating voltages which it may receive from the oscillator over line 28 and circuit I9, 20 and thus tube 2| becomes equivalent to a resistance shunted across l8 and 20. Now when the line is terminated in a reactance and a resistance in parallel it no longer has a negligible effect on the oscillator at point I6 but has the effect of paralleling this part of the oscillator circuit with a reactance. This reactance will change the constants of the oscillating circuit and result in a change in oscillator frequency. As the value of resistance introduced by tube 2| varies from infinity to zero the impedance presented by the line at point l6 will vary from infinity to the surge impedance of the line. This impedance will be equivalent to an inductive reactance varyin between infinity and the surge impedance of the line in parallel with a resistance which will drop from infinity to about twice the surge impedance of the line and then rise again to infinity. Thus, while suificient inductance is introduced into the oscillating circuit to change its frequency materially the resistance introduced will not have an effect suflicient to prevent oscillations.

It should be noted that the line l8 acts as a shunt between the grid 4 of the oscillating circuit and ground. Thus it shunts the series capacity C above, between the grid 4 and ground described above.

Figures 2, 3, and 4 show modifications of the above embodiment of my invention.

In Figure 2 I have added a second line 38 which is connected at point 36 to the control grid 3 of the oscillating circuit. This line thus is shunted across the second series capacity C of the oscillator circuit. It can be made identical with line I8 and it can be connected to circuits identical to circuits Is to 22 inclusive and thus obtain a greater degree of frequency modulation than can be obtained with only the one line and associated apparatus.

If desired connection can be made only at point 36 instead of point l6. This in some cases will be found more practical and desirable.

As illustrated in Figure 2 line 38 has an ef fective length of of a wave .length. The tunable circuit in which it is terminated represented by 33, 40 is tuned for a lower frequency than that of the oscillator and thus it terminates the line in a capacitive reactance. The line is tapped down on 38 to a point where the value of this reactance is equal to the surge impedance of the line. Now when tube 4| is biased beyond cutoff the reactance presented by line 38 at point 36 will be very high and will have a negligible effect on the oscillator. When the bias on tube 4| is varied between cutofi and its working ,value the effective resistance shunted across circuits 39, 40 will vary between infinity and some finite value. This will cause the impedance in which line 38 is terminated to vary between a nearly pure capacity reactance and a relatively low resistance. This in turn will cause the impedance presented by the line at 36 to vary between a very high essentially infinite value and a value of capacity reactance approaching the surge impedance of the line. This impedance will consist of the equivalent of a capacitive reactance shunted by a resistance. The value of this resistance will start with infinity, will drop to a value of about twice the surge impedance 'of the line and will again rise and approach The change in impedance shunted between grid 3 and ground as described above will change the frequency of the oscillator in an opposite direction from that which line |8 changes it as described above. Thus, by differentially biasing tubes 2| and 4| a greater frequency variations can be brought about than would be obtained with either one of these lines and associated circuits.

Figure 2 shows circuits for bringing about this differential biasing effect for telegraphic keying. In this arrangement 4| has its grid 42 connected through resistance 44 to key 48 and through resistances 44 and 41 to tap 52 on a potential divider. Key 48 is used to bias tube 4| in a manner similar to that described above. Tube 2| has its grid 22 connected through resistance 45 to the anode circuit of tube 4| and through resistances 45 and 46 to a point 53 on the potential divider. The cathode of tube 2| is connected to a point 54 on the potential divider while the anode circuit of tube 2| is connected to point 55 on said potential divider. It is noted that the grids of both tubes are negative relative to the cathodes of the respective tubes. When tube 4| is blocked no anode current will fiow so that there will be no potential drop across resistor 46. Thus the bias on tube 2| will be as determined by the setting of points 53 and 54 on the voltage divider 50. These points will be set for a working value of bias, i. e., so that current fiows in tube 2|. Now when'tube 4| is made conducting by opening key 48 current will flow through resistor 46 and through the anode of 4|. This current will set up voltage across 46 and will cause the voltage impressed on the grid of tube 2| to increase and thus to block tube 2|.

Figure 3 shows the manner in which tubes 2| and 4| may be differentially biased when a modulating wave such as that encountered in voice transmission is employed. Here battery 6|] is shown supplying anode voltage to tubes 2| and 4| and supplying working bias voltage to the grids of these tubes through the midtap of the secondary of modulation transformer 6|. The modulation input connected to the primary of this transformer induces voltages on the grids of 2| and 4| which are opposite to each other. That is, when the grid of 2| becomes more negative the grid of 4| becomes less negative, etc. Thus, these two tubes effect their respective lines so as to swing the frequency in the same direction for a particular modulation input current. This type of modulation will be superior to that obtained if only one of these tubes and its associated circuits were used since the distortion introduced tends to cancel.

The modification in Figure 4 is in principle the same as the modifications in the prior figures. The arrangement in Figure 4, however, diifers from the arrangements inthe prior figures in that the line l8 connects the anode l3 and a grid 10 of a tube 12 to the electrode 4 of the oscillation generator I5. "This circuit is completed as in the prior modification by grounding the cathode of the tube 12 and I5. Keying is accomplished by applying a potential to the grid 14 of tube 12 which varies between ground or other point of fixed potential and a higher negaan oscillation generator including an electron discharge tube having electrodes, including a plu rality of grid electrodes, connected in high frequency alternating-current circuits, a frequency determining piezo-electric crystal connected with 75 two of said grid electrodes, an impedance variable at signal frequency, a transmission line connecting said impedance to said crystal, and means for varying said impedance between a high value and a low value whereby-the impedance presented by the line to the crystal varies between a high value and a low value in accordance with variations in said impedance.

2. A frequency modulation system comprising, an oscillation generator including an electron discharge tube having electrodes, including a plurality of grid electrodes, connected in high frequency alternating-current circuits, a frequency determining piezo-electric crystal connected with two of said grid electrodes, an impedance variable at signal frequency, and a transmission line having a terminating reactance which substantially matches its surge impedance connected at one point to said impedance and at another point in shunt to said piezo-electric crystal.

3. A frequency modulation system comprising in combination, an oscillation generator comprising an electron discharge tube having a frequency determining element connected with its electrodes, a reactance tuned to a frequency slightly different than, the frequency of operation of said generator, an impedance variable at signal frequency connected with said reactance, and a transmission line connecting said reactance to said element.

4. A frequency modulation system comprising, an oscillation generator including an electron discharge tube having electrodes, including a plurality of grid electrodes, connected in high frequency alternating-current circuits, a frequency determining piezo-electric crystal connected with two of said grid electrodes, an impedance variable at signal frequency, a transmission line having a terminating reactance which substantially matches its surge impedance connecting said impedance to said element, and an output circuit connected with an electrode of said tube coupled to the oscillation generator electrodes by the electron stream only of the tube.

5. A frequency modulation system comprising, an oscillation generator including an electron discharge tube having electrodes, including a plurality of grid electrodes, connected in high frequency alternating-current circuits, a frequency determining piezo-electric crystal connected with two of said grid electrodes, a resistance variable at signal frequency a reactance in shunt to said resistance and a transmission line an odd number of A; wave lengths long connecting said resistance and reactance to said element.

6. A frequency modulation system comprising, an oscillation generator'including an electron discharge tube having electrodes, including a plurality of grid electrodes, connected in high frequency altemating-current circuits, a frequency determining piezo-electric crystal connected with two of said grid electrodes, an impedance variable at signal frequency substantially between infinity and a relatively low value, and a transmission line having a terminating impedance substantially equal to its surge impedance connected at one end in shunt to said impedance and at the other end in shunt to said piezo-electric crystal, said transmission line being an odd number of A; wave lengths long.

'7. A frequency modulation system comprising in combination, an oscillation generator comprising an electron discharge tube having a frequency determining element connected with its electrodes, a reactance tuned to a frequency slightly difierent than the frequency of operation of said generator, an impedance variable at signal frequency connected with said reactance, and a transmission line an odd number of wave lengths long connecting said reactance to said element.

8. A frequency modulation system comprising in combination, an oscillation generator including an electron discharge tube having a frequency determining piezo-electric crystal connected with its electrodes, impedances variable at signal frequency, and a separate transmission line connecting each of said impedances to said crystal.

9. A frequency modulation system comprising in combination, an oscillation generator comprising an electron discharge tube having a frequency determining element connected with its electrodes, a plurality of reactances each tuned to a frequency slightly different than the frequency of operation of said generator, an impedance variable at signal frequency connected with each of said reactances, and transmission lines connecting each of said reactances to said element.

10. A frequency modulation system comprising in combination, an oscillation generator including an electron discharge tube having a frequency determining element connected with its electrodes, a plurality ofimpedances variable at signal frequency and substantially identical transmission lines connecting each of said impedances to a different point on said element.

11. A frequency modulation system comprising in combination, an oscillation generator including an electron discharge tube having a frequency determining element connected with its electrodes, an impedance variable at signal frequency, a reactance tuned to a frequency of the order of the frequency of operation of said generator connected in. shunt to said impedance and a transmission line connecting said reactance and impedance to a point on said element, said line having an electrical length represented by the formula (4n+1) 7\/8, where n is a whole number including zero and A is the length of the waves generated.

12. A frequency modulation system comprising in combination, an oscillation generator including an electron discharge tube having a frequency determining piezo-electric crystal connected with its electrodes, an impedance variable at signal frequency substantially between infinity and a very low value a transmission line substantially of a wave length long connecting said impedance to said crystal, and a reactancev tuned to a frequency of the order of the frequency of operation of said generator connected to the end of said line at which said impedance is connected.

13. In a frequency modulation system, an oscillation generator of the electron discharge tube having oscillation generating and frequency determining elements interconnecting the electrodes of the tube, a two wire line connected at one end in shunt to one of said frequency determining elements, an impedance connected to the other end of said line, said impedance substantially matching the surge impedance of the line and comprising an inductance and a capacity in parallel and an electron discharge device impedance variable at signal frequency connected to said other end .Of said line.

14. In a frequencymodulation system, an oscillation generator of the electron discharge tube type having oscillation generating and frequency determining elements interconnecting the electrodes of the tube, a transmission line connected with one of said elements and an impedance terminating said line, said impedance comprising an inductance and a capacity connected with the output electrodes of an electron discharge device variable at signal frequency, said inductance and capacity being tuned to resonate at a frequency slightly higher than the frequency of operation of said generator, said line having a length represented by the formula (4n+1) M8 where n equals zero or a whole number and is the length of the oscillations generated.

15.In a frequency modulation system, an oscillation generator of the electron discharge tube type having oscillation generating and frequency determining elements interconnecting the electrodes of the tube, a transmission line connected with one of said elements, animpedance terminating said line, said impedance comprising an inductance and capacity connected with the output electrodes of an electron discharge device variable at signal frequency, said capacity and inductance being adjusted to resonate at a frequency slightly lower than the frequency of operation of said generator and said line having a length represented bythe formula (4n+3) M8 where n is a whole number.

16. In a frequency modulation system an oscil-- lation generator of the electron discharge tube type having electrodes connected in oscillation generating and frequency determining circuits, a first modulating impedance variable at signal frequency, a second modulating impedance variable at signal frequency, a transmission linecoulation generator of the electron discharge tube type having. electrodes connected in oscillation generating and frequency determining circuits, a transmission line coupled to said frequency determining circuit, said transmission line being represented by the formula (4n+1) )\/8, a first terminating impedance comprising an inductance and a capacity and an impedance variable at signal frequency connected to said first transmission line, said inductance and capacity being adjusted to present a negative reactance to said transmission line, a second transmission line connecting said second modulating impedance to said frequency determining circuits, said second transmission line being represented by the formula (4n+3) M8, a second terminating impedance comprising an inductance and a capacity, and an impedance variable at signal frequency connected to said second transmission line, said 35 last named inductance and capacity presenting a positive reactance to its line.

' 18. In a frequency modulation system, an oscillation generator of the electron discharge device type having electrodes connected in carrier 7 frequency oscillation generating circuits including a frequency determining element, an artificial line comprising. inductances and capacity of values such that the line has a length substantially equal to (4n+1) M8 where n is zero or any whole number and A is the wave length of the oscillations generated in said device, means connecting 5 one terminal of said line to said frequency determining element, reactances connected with the other end of said line, said reactances substantially matching the surge impedance of the line and an impedance variable at signal fre- 10 guency connected to the said other end of said ine.

v19. In a modulation system an' electron discharge device generator having its electrodes connected in oscillation generating circuits including 15 a frequency determining element, a modulating device comprising a pair of electron discharge devices having their anodes connected in high frequency circuits and their controlling electrodes connected in push-pull relation to a source of 20 modulating potentials, and a transmission line connecting each of said high frequency circuits to said frequency determining element, said transmission lines each having an electrical length equal to an odd number of A; wave lengths as of the oscillation generated in said first named devices.

20. In a frequency modulation system, an electron discharge tube oscillator having its electrode connected in oscillation generating circuits in- 30 cluding as a frequency controlling element a capacity between certain of the electrodes of said tube, a transmission line connected at one end to shunt said capacity, a circuit tuned to a frequency of the order of the frequency of opera- 5 tion of said tube oscillator, means connecting the other end of said line to said tuned circuit to include reactance thereof suiiicient to substantially match the surge impedance of the line to its terminating reactance formed by said tuned circuit, and an impedance variable at signal frequency through a wide range connected to said tuned circuit.

21. In a system for producing oscillatory energy and for keying the frequency of the oscillatory energy produced, an oscillation generator including an electron discharge tube having eiec-- trodes connected in high frequency circuits including a frequency determining element, a plur-ality of impedances variable at signal frequency, transmission lines connecting each of said impedances to said frequency determining element, and keying means connected with said impedances.

22. In a system for generating oscillations and 55 for keying the frequency of the oscillations generated in accordance with signals, an electron discharge device of the multi-grid type having electrodes connected in alternating-current circuits,

a piezo-electric crystal connected to a pair of 60 said grid electrodes, a plurality of electron discharge tubes each having input and output electrodes, alternating current circuits connected with the output electrodes of each of said tubes, transmission lines connecting said alternating current circuits to said crystal, and means connected with the input electrodes of said tubes for controlling the impedance of the said tubes. in accordance with keying signals.

JAMES L. FINCH'. 

